Subject: + memcg-use-__this_cpu_sub-to-dec-stats-to-avoid-incorrect-subtrahend-casting.patch added to -mm tree
To: gthelen@xxxxxxxxxx,hannes@xxxxxxxxxxx,tj@xxxxxxxxxx
From: akpm@xxxxxxxxxxxxxxxxxxxx
Date: Tue, 29 Oct 2013 14:01:37 -0700
The patch titled
Subject: memcg: use __this_cpu_sub() to dec stats to avoid incorrect subtrahend casting
has been added to the -mm tree. Its filename is
memcg-use-__this_cpu_sub-to-dec-stats-to-avoid-incorrect-subtrahend-casting.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/memcg-use-__this_cpu_sub-to-dec-stats-to-avoid-incorrect-subtrahend-casting.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/memcg-use-__this_cpu_sub-to-dec-stats-to-avoid-incorrect-subtrahend-casting.patch
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From: Greg Thelen <gthelen@xxxxxxxxxx>
Subject: memcg: use __this_cpu_sub() to dec stats to avoid incorrect subtrahend casting
As of v3.11-9444-g3ea67d0 ("memcg: add per cgroup writeback pages
accounting") memcg counter errors are possible when moving charged memory
to a different memcg. Charge movement occurs when processing writes to
memory.force_empty, moving tasks to a memcg with
memcg.move_charge_at_immigrate=1, or memcg deletion. An example showing
error after memory.force_empty:
$ cd /sys/fs/cgroup/memory
$ mkdir x
$ rm /data/tmp/file
$ (echo $BASHPID >> x/tasks && exec mmap_writer /data/tmp/file 1M) &
[1] 13600
$ grep ^mapped x/memory.stat
mapped_file 1048576
$ echo 13600 > tasks
$ echo 1 > x/memory.force_empty
$ grep ^mapped x/memory.stat
mapped_file 4503599627370496
mapped_file should end with 0.
4503599627370496 == 0x10,0000,0000,0000 == 0x100,0000,0000 pages
1048576 == 0x10,0000 == 0x100 pages
This issue only affects the source memcg on 64 bit machines; the
destination memcg counters are correct. So the rmdir case is not too
important because such counters are soon disappearing with the entire
memcg. But the memcg.force_empty and memory.move_charge_at_immigrate=1
cases are larger problems as the bogus counters are visible for the
(possibly long) remaining life of the source memcg.
The problem is due to memcg use of __this_cpu_from(.., -nr_pages), which
is subtly wrong because it subtracts the unsigned int nr_pages (either -1
or -512 for THP) from a signed long percpu counter. When nr_pages=-1,
-nr_pages=0xffffffff. On 64 bit machines stat->count[idx] is signed 64
bit. So memcg's attempt to simply decrement a count (e.g. from 1 to 0)
boils down to:
long count = 1
unsigned int nr_pages = 1
count += -nr_pages /* -nr_pages == 0xffff,ffff */
count is now 0x1,0000,0000 instead of 0
The fix is to subtract the unsigned page count rather than adding its
negation. This only works once "percpu: fix this_cpu_sub() subtrahend
casting for unsigneds" is applied to fix this_cpu_sub().
Signed-off-by: Greg Thelen <gthelen@xxxxxxxxxx>
Acked-by: Tejun Heo <tj@xxxxxxxxxx>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/memcontrol.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff -puN mm/memcontrol.c~memcg-use-__this_cpu_sub-to-dec-stats-to-avoid-incorrect-subtrahend-casting mm/memcontrol.c
--- a/mm/memcontrol.c~memcg-use-__this_cpu_sub-to-dec-stats-to-avoid-incorrect-subtrahend-casting
+++ a/mm/memcontrol.c
@@ -3774,7 +3774,7 @@ void mem_cgroup_move_account_page_stat(s
/* Update stat data for mem_cgroup */
preempt_disable();
WARN_ON_ONCE(from->stat->count[idx] < nr_pages);
- __this_cpu_add(from->stat->count[idx], -nr_pages);
+ __this_cpu_sub(from->stat->count[idx], nr_pages);
__this_cpu_add(to->stat->count[idx], nr_pages);
preempt_enable();
}
_
Patches currently in -mm which might be from gthelen@xxxxxxxxxx are
percpu-fix-this_cpu_sub-subtrahend-casting-for-unsigneds.patch
memcg-use-__this_cpu_sub-to-dec-stats-to-avoid-incorrect-subtrahend-casting.patch
memcg-refactor-mem_control_numa_stat_show.patch
memcg-support-hierarchical-memorynuma_stats.patch
percpu-add-test-module-for-various-percpu-operations.patch
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